1. Field of the Invention
The present invention relates to an optical wavelength division multiplexing transmission system for transmitting multiplexed light formed by multiplexing light rays of a plurality of wavelengths, including transmitting signals, and more particularly to optical repeater equipment to relay optical signals on the transmission line without regenerating electric signals from the optical signals.
2. Related Art
In recent years, the optical transmission systems have spread extensively for reasons, such as wide transmission bandwidth, less transmission loss, immunity to electromagnetic induction disturbances, etc. In the optical transmission system, optical terminal equipment converts an electric signal into an optical signal, and this optical signal passes through generally one or more stages of optical repeater equipment to the opposite terminal equipment where the optical signal is converted into an electric signal. The optical transmission systems are roughly divided into two types according to the method by which the optical repeater equipment amplifies the optical signal. One type is the regenerative repeat method, in which the optical repeater equipment converts an optical signal into an electric signal, and, after amplification, converts this electric signal again into an optical signal to transmit further along the transmission line. The other type is the non-regenerative repeat method, in which the optical repeater equipment amplifies the optical signal and transmits it further along the line without converting the optical signal into an electric signal. Use of optical transmission system of the latter non-regenerative repeat type is now spreading because of advantages, particularly due to their recent practical application of feasible light amplifiers, which contributes to the greater ease of configuring the optical repeater equipment.
The optical transmission system that operates by the conventional non-generative repeat method uses one optical fiber and one light amplifier to transmit one signal along the transmission line, which arrangement is designed to facilitate the maintenance and the management of the optical transmission system.
FIG. 7 is a diagram showing the configuration of the conventional non-regenerative repeat type optical transmission system. In FIG. 7, pieces of optical terminal equipment 3000 has a group of subscribers 1000 and 6000 have respective groups of subscribers 1000, 2000. Pieces of equipment 3000 and 6000 respectively, convert subscriber signals from the subscribers 1000 and 2000 into optical signals with E/O converters, and transmit the optical signals to the other piece of optical terminal equipment 6000 and 3000 respectively through two pieces of optical repeater equipment 4000 and 5000. The pieces of optical repeater equipment 4000 and 5000 amplify the optical signals with their amplifiers and transmit the optical signals to the two pieces of optical terminal equipment 6000 and 3000. The pieces of optical terminal equipment 6000 and 3000 convert the optical signals into electric signals with their O/E converters, and distribute to the subscribers 2000, 1000.
In the non-regenerative repeat type optical transmission system, the pieces of optical repeater equipment 4000 and 5000 each have optical fibers and light amplifiers connected thereto both corresponding to the number n of optical signals to be relayed by the two pieces of optical repeater equipment. By this arrangement, the pieces of optical terminal equipment 3000 and 6000 to supervise the optical transmission system, can obtain the correspondence relationship among the wavelengths (.lambda.1, .lambda.2, . . . , .lambda.n) of light rays to be relayed, the traveling directions of the rays, and the light amplifiers for the rays. For example, therefore it is known that an optical signal of wavelength .lambda.1 travels in the upstream direction through optical fibers 7000-1, 7100-1 and 7200-1, and this optical signal is amplified by the light amplifiers 8000-1 and 8100-1.
Because the above-mentioned correspondence relationship is grasped, even if a failure should occur in the optical fiber or the light amplifier, it is easy to replace the faulty optical fiber or light amplifier, or switch the transmission line, in which the failure has occurred, to the spare transmission line. Since normally the optical repeater equipment is an unmanned unit, it is required that information about the optical repeater equipment should be collected at a monitoring station in manned optical terminal equipment, or in supervisory equipment installed at a place remote from the optical transmission system. For this purpose, one supervisory control signal is allocated to one transmission line, and this supervisory control signal conveys information about the pieces of optical repeater equipment along the line to the monitoring station. For example, with the line of wavelength .lambda.1, a supervisory control signal sv-1 conveys information about a light amplifier 8000-1 of optical repeater equipment 4000 and a light amplifier 8100-1 of optical repeater equipment 5000 to the monitor station.
In the conventional optical transmission system, however, when additional transmission lines are to be laid, in other words, when additional optical fibers and additional light amplifiers are added on, it is necessary to prepare circuits to receive and transmit supervisory control signals in proportion with the number of transmission lines additionally to be installed. For example, in the optical transmission system shown in FIG. 7, if three transmission lines are to be added, it is required to prepare 6 (3.times.2) circuits, including those circuits by which to receive and transmit supervisory control signals. As described above, a problem with the optical transmission system is that the cost of additional installation amounts to a large sum.
Meanwhile, wavelength division multiplexing (WDM), by which multiple wavelength signals are multiplexed along a single optical fiber, is becoming popularly employed in practical applications. By using this wavelength division multiplexing technique, it is possible to transmit a larger number of signals on a single optical fiber than the conventional optical transmission system described above. From a viewpoint of maintenance and management, it is still necessary to detect the correspondence relationship of the wavelengths of optical signals, transmission directions, transmission lines, optical amplifiers, etc. Despite this necessity, a technique to efficiently concentrate information obtained at the optical repeater equipment into the supervisory equipment has not been established.